This invention relates to a dual temperature absorption refrigeration system for a vehicle and more particularly for use in commercial vehicles such as refrigerated trucks for transporting perishable commodities.
Wide-spread efforts are currently under way to develop "drop-in" replacements for common forms of halogenated hydrocarbon refrigerants (CHC's) to overcome stratospheric ozone depletion problems, but permit existing types of vapor compression refrigeration to continue in service.
Principal sources of escaping CHC refrigerants are vehicle air-conditioning systems. While "drop-in" replacements might be developed for these systems, use of vehicular vapor-compression refrigeration systems themselves encumber a substantial consumption of automotive fuels to deliver shaft power supplying compressors of these air-conditioning systems.
Public attention has also been attracted to need to conserve petroleum fuel resources and need to reduce vehicular air pollution resulting from its consumption in motor vehicles. Approximately 15% of fuel consumed by air-conditioned vehicles is caused by power demands of air-conditioning compressors driven from crankshaft power to supply air-conditioning demand.
In refrigerated truck cargo applications, more than power to provide comfort air-conditioning for the driver is required by concurrent need to provide refrigeration for cargo. Many frozen foods must be transported while maintaining them in a sub-freezing condition, and many dairy products and beverages, while not requiring freezing conditions, must be maintained in transit in a chilled condition. Refrigeration equipment to supply that cargo requirement also depends upon petroleum fuel consumption to drive refrigeration compressors. In such systems, refrigeration temperatures required for the differing demands require two different evaporator pressures to effect.
Refrigeration systems supplying refrigeration at more than one temperature have been used in known vapor-compression refrigeration applications, and have been accomplished by the use of multiple effect compressors in which lower pressure refrigerant vapor enters the compressor farther down in its compression cycle while higher pressure vapor enters farther up, with both exiting the compressor at a common exit pressure.
Cascade absorption refrigeration systems have also been constructed in the prior art employing multiple absorbers and evaporators arranged in series. An upper cycle provides lowered temperatures for the absorber in a succeeding stage, permitting lower operating pressure in the succeeding absorber and evaporator, such a cascaded set serving to permit achieving far lower ultimate refrigeration temperatures in the final stage than could have been realized by a simple cycle alone. In one such ammonia/water absorption refrigeration cascade system built in Germany prior to World War I, the final evaporator stage of the cascade delivered refrigeration at a temperature below -90.degree. F. (-67.degree. C.), inferring an evaporator and absorber operating pressure in the final stage in the high vacuum range of approximately 1.7 psia or lower. The end product was that final refrigeration temperature.
There are essentially three general types of evaporators employed in the refrigeration industry. The "flooded" and "wet expansion" evaporators require maintenance of a controlled liquid level of refrigerant in the evaporator. The "dry expansion" ("once-through" type) operates with a variable liquid volume of refrigerant in the evaporator, is simpler and less expensive, and is generally employed where the installation is subject to vibration or movement (viz.--aboard ships and aircraft).
In excess of thirty percent of the heat energy value of automotive fuels consumed is dissipated as waste heat from vehicle engine exhausts and cooling systems. A very old form of refrigeration system, invented in France shortly before the American Civil War, commonly known as "absorption refrigeration" (AR), does not depend on a mechanical vapor-compression process. The process involves a thermodynamic refrigeration cycle in which: absorption of a refrigerant in an absorbent at low pressure occurs in an "absorber" vessel; pressurizing the resulting solution (the strong refrigerant/absorbent liquor); heating the strong pressurized liquor from an external heat source to evaporate off refrigerant vapor in the "generator" vessel (a distillation process); condensing the refrigerant vapor so liberated by external cooling in the "condenser" vessel; and releasing the high pressure refrigerant condensate through a pressure reducing device to cause revaporization of the refrigerant with heat of vaporization being absorbed from whatever is being refrigerated in the "evaporator" vessel (the same evaporative cooling process that occurs in vapor-compression refrigeration cycles).
At that point, weak high pressure refrigerant/absorbent solution residue left in the generator, after refrigerant has been distilled off, is returned from the generator via a pressure reducing device, recombined with low pressure refrigerant vapor returned from the evaporator, and cooled in the absorber vessel to reconstitute the strong liquor solution and repeat the cycle.
The oldest form of such systems employed ammonia as refrigerant and water as absorbent. Ammonia contains no halogen atoms at all, and even its gradual leakage into the atmosphere poses no environmental or atmospheric risks. By employing waste heat discharged from vehicle internal combustion engines as an external input energy source to an ammonia AR system (AAR) operating on an ammonia/water combination, the engine shaft can be relieved of the load created by power demand to drive the compressor of a vapor-compression refrigeration system, and all risk of discharging CHC's can be eliminated.
The basic concept of employing AR systems for automotive application has been reported in the prior art dating back to the 1920's. Little has been done to develop the concept due to the convenience and simplicity with which small compressors could be installed as component auxiliaries under the hood of the engine compartment of automotive vehicles, and the simplicity of their installation along a vehicle assembly line.
Another difficulty has been limitation on development of suitable operating cycles because of temperature variation of available waste heat sources from vehicular internal combustion engines.
Exhaust heat offers a higher temperature heat source for AR system external energy supply. Exhaust temperatures are highly variable depending on engine loading. Diesel engine exhaust may be in the 400.degree. to 500.degree. F. (205.degree. to 260.degree. C.) range at 75% load and drop to the 200.degree. to 300.degree. F. (93.5.degree. to 149.degree. C.) range during idling. Use of pollution control exhaust devices also affects exhaust temperatures. Thermally controlled pollution abatement systems may operate close to the engine exhaust outlets at temperatures in the 1500.degree. F. (815.degree. C.) range. Catalytic systems, placed farther along the exhaust system operate in the 800.degree. to 1,000.degree. F. (427.degree. to 538.degree. C.) range.
Examples of total heat energy loss from internal combustion engines removed by coolant circulation include 20% to 35% for automotive diesels and up to 40% at one-third load. Brake horsepower fuel value heat losses amount to 40 to 50% for large diesels and 100 to 150% for automotive engines.
Other than peak temperature sources available as input to an AR cycle, ability of an AR system to operate effectively is also constrained by coolest ambient temperature available for operation of refrigerant condensers and removal of waste heat rejected from absorber vessels. This is ultimately related to temperature of ambient air. For vehicle refrigeration systems to be capable of reliable year round operation in any part of the country, this may require use of a design cooling air temperature at least as hot as the hottest ambient air through which a vehicle may be expected to operate.
Typical vehicle radiators are designed on the basis of a 100.degree. F. (55.6.degree. C.) temperature gradient between ambient air and circulating coolant fluid temperature. The cooling effect created normally results in a temperature drop in the coolant between 35.degree. and 40.degree. F. (19.4.degree. and 22.2.degree. C.) It is possible that with minimum design temperatures for condenser and absorber cooling being limited by mid-afternoon temperatures in Death Valley, and peak engine heat source temperatures limited by inability of engine lubricants to withstand temperatures much above those cited, the remaining temperature gradient available across which an AAR system might be operated was in a range making further development in use of ammonia/water AR cycles too difficult to permit design of practical systems.
Wide variation in engine exhaust temperatures in response to load variation was also too great to permit reliable control of the heat source to supply an AR system, with wide fluctuations in radiator coolant fluid temperatures creating additional problems with use of that coolant as an AAR system heat sink.
Recent evolution of sophisticated solid state electronic control systems now permits a variety of variables to be controlled reliably, instantaneously, and continuously in service. The enormous amount of waste heat energy available suggests that its use as input to an automotive AR system can permit such a system to supply comfort air-conditioning for personnel and refrigeration for cargo. Starting with a "free" source of energy for refrigeration, additional applications may be developed to use it to: improve lubrication cooling; adapt exhaust pollution control devices to their optimum operating temperatures; etc.,--all powered at no fuel energy consumption cost by a waste heat energy source otherwise being discarded.
In addition, recent developments in communications technology permit manufacturing to be undertaken in response to identifiable "niche" market requirements. There is more than enough refrigerated cargo moving in the United States that does not traverse Death Valley at mid-afternoon in mid-summer to justify producing units capable of reducing fuel consumption for all remaining refrigerated trucking services even if they were not suitable for trans-desert application.
There are essentially four fluid stream flows available for integration with an AAR system: the engine coolant, varying between its temperature at entry to the radiator and its temperature on leaving the radiator; engine lubricant flow, varying between its temperature leaving the engine and its temperature leaving a lube-oil cooler if present; engine exhaust stream, varying between its temperature leaving the exhaust manifold, leaving an intermediate pollution control "converter", and leaving the muffler, and in response to changes in engine power demand; and the fourth stream being flow of ambient air, around and past the vehicle.
Vehicle radiators for truck Diesel engines may be generalized as operating with an assumed temperature differential of approximately 110.degree. F. (61.degree. C.) between coolant and ambient air. A 30.degree. to 40.degree. F. (17.degree. to 22.degree. C.) temperature drop normally occurs in the coolant as it passes through the radiator. The thermostat in the coolant line to the radiator is commonly set to open at about 190.degree. F. (88.degree. C.). That would define current generalized operating temperature constraints for selection of AAR subsystem refrigerant condenser and absorber when use of a circulating engine coolant stream is being considered as heat sink for an AAR cycle. Gasoline engines may operate somewhat at variance with these figures, but the principle remains similar in establishing AAR subsystem design criteria.